1. Field of the Invention
The present invention relates to a moving picture compression/decompression system and more particularly to a method for concealing errors in a moving picture compression/decompression system.
2. Discussion of Related Art
Utilizing digital signal processing technology, many systems have been proposed to compress and transmit a large amount of moving picture information through a transmission channel of a limited bandwidth. In moving picture standards such as MPEG, H.261 and H.263, re-synchronization due to channel errors is performed in units of a slice. Each slice consists of a plurality of macro blocks and each macro block is the unit for motion compensation decoding. A single frame generally includes many slices depending upon the size of an image. When an error is generated in such a slice, all information in the slice with the error is lost.
Also, moving picture coding methods use both variable length coding and motion compensation coding. In such case, an error in a bit of a bit stream results in the loss of a considerable portion of the picture information and continuously affects the frames following. Accordingly, a severe deterioration in picture quality results when a decoder decodes a bit stream with an error. To solve this problem, numerous methods for error concealment using neighboring information to restore the lost information have been proposed.
Typical error concealing methods include a temporal prediction error concealing method and a spatial prediction error concealing method. The temporal prediction error concealing method compensates a lost picture block using a motion vector in the time domain. Referring to FIGS. 1A and 1B, the temporal prediction error concealing method replaces a lost macro block E1 of a current frame CF by a macro block Exe2x80x21 of a previous frame PF located in the same position. Although this method is relatively simple and can quickly compensate a lost block, the accuracy of the compensated block is not high. The temporal prediction concealing method especially results in low performance when there is fast motion or irregular motion between two continuous frames.
To obtain more accurate motion vectors for the temporal prediction error concealing method, a block boundary matching method has been recently used. This method utilizes the boundary pixels surrounding the lost macro block for block matching. Although performance is satisfactory when there is no large change in pixel values across the boundary, the block boundary matching does not result in satisfactory performance at the edge or corner of the picture. Moreover, for interlaced sequence, the conventional methods result in poor performance because spatial correlation is low between fields having different parities. Especially, when there is a scene change, the visual picture quality deteriorates significantly.
The spatial prediction error concealing method compensates a lost picture block by the interpolation of the neighboring blocks in the spatial domain. This method performs a spatial interpolation for a lost block E1 using neighboring blocks C1, C2 and C3 of the current frame CF. Because a linear interpolation results in a deterioration of the resolution near the edge, interpolation filtering according to a direction component of the edge is employed. However, such interpolation requires an apparatus for extracting and distinguishing each direction component of the edge, and requires a significant amount of calculation as compared to the conventional linear interpolation filtering.
Still another error compensation method is based on projection and guarantees good picture quality performance when the damaged macro block is sufficiently smaller than the picture. However, a great amount of calculation at the receiver is required.
Particularly, when image sequences have a strong temporal correlation, the performance of the spatial prediction concealing method is inferior to that of the temporal prediction concealing method. However, when a severe variation between pictures such as a scene change occurs, the spatial prediction compensation is preferable to the temporal prediction compensation. Therefore, a method for deciding whether the information of lost macro block E1 should be compensated by a spatial prediction method using surrounding blocks C1-C4, or replaced by the information of macro block E1 that is the most analogous to the lost macro block E1 is required.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide a method for accurately and efficiently concealing errors, thereby preventing deterioration of the picture quality at the receiver.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following, or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, a method for concealing errors includes extracting the type of a frame in which an error is currently generated and extracting the characteristics of the picture; selecting an error concealing method to use according to the frame type and picture characteristics; replacing the frame with the error by a block in which errors are concealed, according to the judged error concealing method; and performing a post-process for the replaced block, to remove visual block deterioration generated between neighboring blocks.
A spatial prediction error concealing method is selected when the frame with an error has a weak temporal correlation, such as a scene change, and a temporal prediction error concealing method is selected when there is a strong temporal correlation in the frame. Also, the spatial prediction error concealing method uses a frequency interpolation method by which blocks surrounding the lost block are interpolated in the frequency domain in the current frame using a spatial correlation between the blocks surrounding the lost block.
The frequency interpolation method according to the present invention comprises dividing each block surrounding a lost block into subblocks and DCT-converting the subblocks neighboring the lost block; re-dividing the DCT-converted subblocks and re-DCT-converting the re-divided subblocks to estimate the average value of the lower frequency band coefficients; and reverse DCT-converting the estimated average value of the coefficients into subblocks and estimating the lost block to reconstruct it.
In the present invention, the temporal prediction error concealing method uses a new cost function to estimate a block from a previous frame that would most closely agree with the lost block in order to compensate the error.